Extracellular calcium homeostasis is precisely regulated by the interaction of several hormones with multiple target organs. In addition, calcium itself acts upon a G-protein-coupled "calcium- sensing" receptor (CaSR) on the he surface of parathyroid cells to regulate the secretion of parathyroid hormone (PTH). This receptor is also expressed in other tissues, where its precise function is less well-defined. Humans with two copies of an inactivating mutation in the caSR gene are severely hypercalcemic, presumably because the normal inhibition of PTH secretion by calcium is no longer present. CaSR is expressed by diverse cell types of the kidney, where extracellular calcium has multiple effects. The purpose of this proposal is to clarify the role of CaSR in mediating specific effects of calcium on the kidney and to better define the role of renal CaSR in regulating whole animal calcium homeostasis. The aims of this proposal focus on the development of genetically altered mice which will serve as models in understanding the function of renal Car. Transgenic mice overexpressing CaSR in the thick ascending limb of the kidney will be generated. This mouse model should help clarify the contribution of CaSR activation to the regulation of calcium, sodium, and water homeostasis. It will also aid in defining which actions of calcium on the kidney are mediated by CaSR. In addition, the renal function of mice with a null mutation in CaSR will be investigated by a combination of genetic and physiologic methods. Normally these ~knockout~ mice do not live past the neonatal period. Crossing CaSR deficient mice with other mutant mice unable to mount a hypercalcemic response to PTH should generate viable CaSR-deficient mice which will aid in the investigation of CaSR function in the kidney and other tissues. Multiple hypotheses regarding the role of CaSR in renal calcium excretion and as well as its role in mediating renal responses to calcium will be investigated. Clarifying the functions of CaSR, a newly identified component of the calcium homeostatic system, will likely have implications for understanding and treatment of diseases of abnormal calcium regulation, including osteoporosis, hyperparathyroidism, kidney stones, and hypertension.